The present invention relates to the operation of fluid ejection printheads such as inkjet printers or the like and, in particular, discloses a method of providing for thermal compensation for variations in required ejection energies.
Many different types of printing have been invented, a large number of which are presently in use. The known forms of printers have a variety of methods for marking the print media with a relevant marking media. Commonly used forms of printing include offset printing, laser printing and copying devices, dot matrix type impact printers, thermal paper printers, film recorders, thermal wax printers, dye sublimation printers and inkjet printers both of the drop on demand and continuous flow type. Each type of printer has its own advantages and problems when considering cost, speed, quality, reliability, simplicity of construction and operation etc.
In recent years, the field of inkjet printing, wherein each individual pixel of ink is derived from one or more ink nozzles has become increasingly popular primarily due to its inexpensive and versatile nature.
Many different techniques on inkjet printing have been invented. For a survey of the field, reference is made to an article by J Moore, xe2x80x9cNon-Impact Printing: Introduction and Historical Perspectivexe2x80x9d, Output Hard Copy Devices, Editors R Dubeck and S Sherr, pages 207-220 (1988).
Inkjet printers themselves come in many different types. The utilisation of a continuous stream ink in inkjet printing appears to date back to at least 1929 wherein U.S. Pat. No. 1,941,001 by Hansell discloses a simple form of continuous stream electrostatic inkjet printing.
U.S. Pat. No. 3,596,275 by Sweet also discloses a process of continuous inkjet printing including the step wherein the inkjet stream is modulated by a high frequency electrostatic field so as to cause drop separation. This technique is still utilized by several manufacturers including Elmjet and Scitex (see also U.S. Pat. No. 3,373,437 by Sweet et al)
Piezo-electric inkjet printers are also one form of commonly utilized inkjet printing device. Piezo-electric systems are disclosed by Kyser et. al. in U.S. Pat. No. 3,946,398 (1970) which utilizes a diaphragm mode of operation, by Zolten in U.S. Pat. No. 3,683,212 (1970) which discloses a squeeze mode of operation of a piezo-electric crystal, Stemme in U.S. Pat. No. 3,747,120 (1972) discloses a bend mode of piezo-electric operation, Howkins in U.S. Pat. No. 4,459,601 discloses a piezo-electric push mode actuation of the inkjet stream and Fischbeck in U.S. Pat. No. 4,584,590 which discloses a sheer mode type of piezo-electric transducer element.
Recently, thermal inkjet printing has become an extremely popular form of inkjet printing. The inkjet printing techniques include those disclosed by Endo et al in GB 2007162 (1979) and Vaught et al in U.S. Pat. No. 4,490,728. Both the aforementioned references disclose inkjet printing techniques which rely upon the activation of an electrothermal actuator which results in the creation of a bubble in a constricted space, such as a nozzle, which thereby causes the ejection of ink from an aperture connected to the confined space onto a relevant print media. Printing devices utilizing the electro-thermal actuator are manufactured by manufacturers such as Canon and Hewlett Packard.
Most of these devices obviously involve the ejection of the fluid on demand. The ejection of the fluid requires a certain amount of energy depending upon the inkjet device utilized. Unfortunately, the utilization of a particular device will be under varying physical circumstances. For example, the density, specific heat capacity, viscosity, thermal conductivity and surface tension will vary with varying temperatures, sometimes by orders of magnitude. For example, there is a substantial variation in water viscosity with temperature. Where a water based ink is used it is likely that a similar response will be present in ink. Of course, with inks of varying compositions, different values will be relevant. The variation in these parameters can produce substantial fluctuations in the operation of an inkjet device. For example, substantial fluctuations can occur in the energy required to eject a single drop.
It is an object of the present invention to utilize and manipulate the temperature operating conditions of an inkjet printing device so as to provide for advantageous operations.
In accordance with a first aspect of the present invention, there is provided a method of operating a page width ink jet printhead within a predetermined thermal range so as to print an image, said printhead comprising:
an array of nozzles formed on a substrate, each nozzle including a nozzle opening, an associated displaceable thermal actuator for ejecting ink through said nozzle opening, an ink chamber and an activation unit for controlling operation of said actuator;
at least one temperature sensor attached to said substrate for sensing the temperature of said substrate;
a temperature determination unit connected to said at least one temperature sensor; and,
an ink ejection drive unit coupled to said temperature determination unit and to said printhead;
said method including the steps of:
(a) sensing the temperature of said substrate with said at least one temperature sensor and said temperature determination unit;
(b) said ink ejection drive unit determining if said temperature is below a predetermined threshold;
(c) if said temperature is below said predetermined threshold, performing a preheating step of heating said actuators so that the printhead is heated to a temperature above said predetermined threshold;
(d) controlling said preheating step such that said thermal actuators are heated by pulses of energy less than 160 nJ, which are insufficient to cause the ejection of ink from said printhead, ejection of one ink drop from one said nozzle requiring at least 160 nJ; and,
(e) utilizing said printhead to print said image.
The step (a) preferably further includes the steps of: (aa) initially sensing an ambient temperature surrounding the printhead; and, (ab) setting the predetermined threshold to be the ambient temperature plus a predetermined operational factor amount, the operational factor amount being dependent on the ambient temperature.
The method may further comprise the step of: (e) monitoring the printhead temperature whilst printing the image and where the temperature falls below the predetermined threshold, reheating the actuators to again raise the temperature of the printhead above the predetermined threshold.
The step (b) may comprise constantly monitoring the printhead temperature whilst heating the printhead.
The method preferably further includes the step of sensing the resistivity of a heater element associated with each actuator and adjusting a width of the energy pulses to maintain a constant energy of the pulses to compensate for a variation in resistivity of the heater element with temperature.
Suitably, ejection of one ink drop from one nozzle requires between 160 and 190 nJ.
In accordance with a further aspect of the present invention, there is provided a page width ink jet printhead comprising:
an array of nozzles formed on a substrate, each nozzle including a displaceable thermal actuator for ejecting ink on demand through a nozzle opening of its associated nozzle;
an activation unit for each nozzle for controlling operation of said actuators;
at least one temperature sensor attached to said substrate for sensing the temperature of said substrate;
a temperature determination unit connected to said at least one temperature sensor;
an ink ejection drive unit connected to said temperature determination unit and to said printhead;
wherein, before an ink ejection operation is begun, said temperature determination unit utilizes an output from said at least one temperature sensor to sense a current temperature of said substrate, and if said temperature is below a predetermined threshold, said ink ejection drive unit outputs a preheat activation signal to generate pulses of energy of less than 160 nJ to heat each said thermal actuator to an extent sufficient to heat said substrate, while being insufficient for the ejection of ink from said array, ejection of one ink drop from one said nozzle requiring at least 160 nJ.
Suitably, a plurality of spaced apart temperature sensors are formed on the substrate.
Preferably, the array of nozzles is divided into a series of spaced apart groups with at least one temperature sensor per group.
Suitably, the ejection of one ink drop from one said nozzle requires between 160 and 190 nJ.
Preferably, the printhead further comprises at least one pad for sensing the resistivity of a heater element associated with each actuator to enable compensation for a variation in resistivity of the heater element with temperature.
Preferably, each activation unit comprises a heater element external to the ink chamber of each nozzle for heating the actuator.